DESCRIPTION This proposal is designed to explore the prevalence and magnitude of tonic descending inhibition (TDI) impinging on individual lumbar sensory tract neurons. This research will be performed in the intact unanesthetized cat for the first time, and is expected to establish whether TDI is a functional physiological mechanism or merely an epiphenomenon of anesthetic agents and/or surgical procedures utilized in the acute experiments that have demonstrated TDI. The PI will test the long-standing hypothesis that the excitability of L7 postsynaptic dorsal column (PSDC) pathway, spinoreticular tract (SRT), and spinothalamic tract (STT) neurons is tonically suppressed during the state of wakefulness by supraspinal influences and to directly assess the effects of commonly used anesthetics on TDI. This goal will be accomplished by examining the spontaneous and peripherally evoked responses of antidromically identified L7 PSDC, SRT and STT neurons recorded in chronically instrumented cats before, during and following reversible cold block at the L3 spinal cord as well as during general anesthesia. The proposed studies are exploratory in nature in that they address fundamental questions in basic neuroscience that have eluded direct analysis due to inherent technical difficulties and require novel experimental approaches. The high-risk elements associated with this proposal are the technical issues pertaining to obtaining a cold block of the spinal cord in the chronically instrumented cat while recording from the target sets of neurons.